you showed no diagram or any picture / screenshot
By definition we have that the volume is given by
V = h * w * l
where
V: Volume
h: height
w: width
l: length
Clearing the height we have:
h = V / (w * l)
h = (5 - x) / ((x) * (x - 2))
The expression is not defined for the values of x that make the denominator zero:
((x) * (x - 2)) = 0
x = 0
x = 2
Answer
h = (5 - x) / ((x) * (x - 2))
x = 0
x = 2
15% of gross = 630
0.15 G = 630
Divide each side by 0.15 :
G = 630/0.15 = $4,200